This invention is directed to the removal of nitrogen oxides, NO., from the exhaust gases of internal combustion engines, particularly diesel engines, which operate at combustion conditions with air in excess of that required for stoichiometric combustion, i.e., lean. It is well known that fuel efficiency improvements in excess of 10% can be achieved in gasoline engines operated at "lean burn" conditions when compared to today's engines which cycle the air to fuel ratio about stoichiometric. Diesel engines, by their nature, operate at lean conditions and have always achieved high fuel efficiencies.
There are numerous ways known in the art to remove NO.sub.X from a waste gas. This invention is directed to a catalytic reduction method for removing NO.sub.X. A catalytic reduction method essentially comprises passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert NO.sub.X into nitrogen. Two types of catalytic reduction are practiced. The first type is non-selective catalyst reduction and the second type is selective catalyst reduction (SCR). This invention relates to SCR systems.
In the selective catalyst reduction method, a reducing agent or reductant is supplied to the exhaust stream and the mixture is then contacted with a catalyst. Typical reducing agents used in industrial processes for removal of NO.sub.X from waste streams, such as urea or ammonia, are not suited for vehicular applications because of the large quantities of the reducing agent required. Any SCR method using a separate reducing agent is simply not practical for vehicle applications. Additional storage tanks have to be provided. Also, measures have to be taken to account for the environmental affects on the reducing agent such as freezing. In addition, space limitations for the reactor have to be addressed.
The prior art has recognized the limitations present in supplying a separate reducing agent for converting NO. and has used diesel fuel itself as the reducing agent. In U.S. Pat. No. 5,343,702 to Miyajima et al., issued Sep. 6, 1994, excess fuel is injected into the combustion chambers of the engine on a sensed demand basis to provide unsaturated HC to the catalyst. In U.S. Pat. No. 5,189,876 to Hirota et al., issued Mar. 2, 1993, diesel fuel is injected into a cracking chamber formed in the exhaust manifold of the engine. In U.S. Pat. No. 5,788,936 to Subramanian et al., issued Aug. 4, 1998, a wide variety of reductants is introduced into the exhaust stream upstream of an acidic alumina catalyst which is said to be effective to reduce NO.sub.2 to N.sub.2. In U.S. Pat. No. 5,586,433 to Boegner et al., issued Dec. 24, 1996, a separate cracking unit for diesel fuel is supplied. The '702 and '876 patents simply inject diesel fuel into the system. Systems injecting diesel fuel, in liquid form, into the exhaust stream must include provisions for warming the fuel to overcome low temperature conditions encountered in a vehicular environment. In contrast, the '433 patent separately cracks diesel fuel with air and will produce a more reactive reducing agent. A somewhat similar concept to the '433 patent appears to be disclosed in PCT application WO 97/01697 published Jan. 16, 1997. Still further, the catalytic reduction article published by Elsevier Science, B. U., shows admission of fuel and air over a partial oxidation catalyst to form acetaldehyde and formaldehyde as a reductant.
Recently plasma techniques have been proposed for use in emission systems. The method of using plasma techniques can take various forms such as microwave, RF, and even glow discharge. However, many systems utilize corona discharge techniques to generate the plasma. Initially, it was believed that the plasma would reduce NO.sub.X to elemental nitrogen, N.sub.2. (See, for example, U.S. Pat. No. 5,746,984 to Hoard, issued May 5, 1998, using a non-thermal plasma to reduce NO.sub.X and U.S. Pat. No. 5,715,677 to Wallman et al., issued Feb. 10, 1998 using a thermal plasma. See also U.S. Pat. No. 5,440,876 to Bayliss et al., issued Aug. 15, 1995) However, as explained in U.S. Pat. No. 5,711,147 to Vogtlin, issued Jan. 27, 1998, the plasma converts NO.sub.X to NO.sub.2 which must then be subsequently reduced by a selective catalyst.
The NO.sub.X plasma art has also utilized a reducing agent with the plasma. In U.S. Pat. No. 5,807,466 to Wang et al., issued Sep. 15, 1998, diesel fuel is injected upstream of a NO.sub.X plasma reactor to produce N.sub.2. In U.S. Pat. No. 5,767,470 to Cha, issued Jun. 16, 1998, diesel fuel is injected into the exhaust gas prior to entering a two stage microwave converter. In the '147 patent, diesel fuel is passed through a plasma prior to mixing with the exhaust gas and the entire mixture is passed through a second plasma reactor. See also European patent application 0 366 876 B1 published May 12, 1993, and the '876 patent for injecting other reducing agents, i.e., ammonia, into the exhaust gas passing through the plasma.
All of these approaches pass the entire exhaust stream with or without a reducing agent through a plasma reactor. Energy is required to generate the plasma and because the entire exhaust stream is treated by the plasma, a significant amount of energy must be developed by the vehicle to produce the plasma. Early plasma units placed an electrical load on the vehicle approximately equal to a 10% fuel penalty. Further, the load is a constant horsepower drain. Thus, with the vehicle cruising or idling at minimal engine fuel usage, the drain to operate the reactor represents a higher fuel utilization than the 10% value. It makes little sense to operate the engine at a lean burn condition to produce an overall fuel savings of about 10% if that savings is used by the energy required to power a reactor to eliminate NO.sub.X emissions which are in significant quantities because the engine is operated lean. Recent developments in the reactors have led to published literature claims of less energy utilization. The fact remains that the entire exhaust gas stream must be passed through the plasma which must be sufficiently energetic to induce reactions with the gases passing through the plasma. The prior art plasma NO.sub.X reduction systems are inherently energy inefficient systems.
Apart from using the plasma to react with NO.sub.X in a manner which converts the NO.sub.X or allows further conversion of the NO.sub.X to N.sub.2, plasma has been generated in air to react with or oxidize coke inherently produced by diesel engines. The '677 and the '876 patents illustrate plasma reactors burning soot with air introduced in a metallic bed situated in a plasma field, i.e., thermal plasma or a ferro-electric bed in the plasma, i.e., non-thermal plasma.